Liquid crystal display devices employ optical anisotropy and dielectric anisotropy of liquid crystal substances, and display mode of the devices includes TN (twisted nematic) mode, DS (dynamic scattering) mode, G.H (guest.host) mode, DAP (deformation of aligned phases) mode, and STN (super twisted nematic) mode. In order to cope with the request for higher display quality, the demand for display devices of active matrix type a typical example of which is thin film transistor (TFT) has recently been increased.
While properties required of liquid crystal substances are different from the mode to mode, liquid crystal substances are necessary to be stable against moisture, air, heat, and light in any display mode. Besides, the substances should exhibit liquid crystal phase in a temperature range as wide as possible with room temperature being its center, should have a low viscosity, excellent miscibility with other liquid crystalline compounds, large dielectric anisotropy value (.DELTA..epsilon.), and most desirable optical anisotropy value (.DELTA.n), and should be chemically and electrically stable. Particularly, a high voltage holding ratio is required of display devices of active matrix type. However, compounds which completely satisfy such requirements as mentioned above by a single compound have not been found up to now, and thus it is an actual situation that liquid crystal compositions obtained by mixing several kind of liquid crystal compounds or liquid crystalline compounds are being used.
Recently, liquid crystal display devices which can be driven at a low voltage and have a high response speed are sought for TFT type liquid crystal devices. Accordingly, development of liquid crystal materials exhibiting a high voltage holding ratio, and having a high .DELTA.n and high .DELTA..epsilon. is actively being carried out.
Heretofore, it is known that fluorine substituted liquid crystal materials exhibit a high voltage holding ratio, and that liquid crystalline compounds having a plural number of aromatic rings have a high .DELTA.n. It is also known that compounds in which aromatic ring is pyrimidine ring have a high .DELTA..epsilon.. Based on these knowledge, it can be expected that liquid crystalline compounds which have pyrimidine ring and fluorinated phenyl ring have a high .DELTA.n and a high .DELTA..epsilon., and are also high in voltage holding ratio. In Laid-open Japanese Patent Publication No. Hei 2-233626, a liquid crystalline compound (compound expressed by the formula 10) having pyrimidine ring and partially being replaced by fluorine atom is disclosed. ##STR2##
Voltage holding ratio of the compound (formula 10) was 97.5% at 25.degree. C. and 92.9% at 100.degree. C., synthesis and determination of voltage holding ratio of which were carried out by the present inventors according to the description in Laid-open Japanese Patent Publication No. Hei 2-233626. Besides, physical properties of the compound (formula 10) which was calculated by extrapolation from physical properties both of (a) the composition obtained by dissolving the compound (formula 10) in a mother liquid crystal having nematic phase and (b) the mother liquid crystal itself, and their mixing ratio were as follows:
N-I transition point 83.7.degree. C. .DELTA..epsilon. 18.7 .DELTA.n 0.204
Whereas this compound has a comparatively high voltage holding ratio, its .DELTA..epsilon. value is insufficient for coping with the request for driving liquid crystal display devices at a low voltage. Besides, the compound (formula 10) has a defect, as shown in Comparative Examples described below, that miscibility with other liquid crystalline compounds at low temperatures is poor. Since the mixing ratio of liquid crystal compounds having a poor miscibility at low temperature in liquid crystal compositions can not be increased, it is impossible to make the characteristics of such compounds contribute to the improvement of characteristics of liquid crystal compositions even if the compounds had some other excellent characteristics.
An object of the present invention is to provide liquid crystalline compounds which have a high voltage holding ratio, high .DELTA.n, and high .DELTA..epsilon., and are excellent in miscibility with other liquid crystalline compounds at low temperatures; to provide liquid crystal compositions comprising the liquid crystalline compound; and to provide liquid crystal display devices fabricated by using the liquid crystal composition.